Somatostatin (SRIF), a tetradecapeptide discovered by Brazeau et al., has been shown to have potent inhibitory effects on various secretory processes in tissues such as pituitary, pancreas and gastrointestinal tract. SRIF also acts as a neuromodulator in the central nervous system. These biological effects of SRIF, all inhibitory in nature, are elicited through a series of G protein coupled receptors, of which five different subtypes have been characterized (sstr1-sstr5). These five subtypes have similar affinities for the endogenous SRIF ligands but have differing distribution in various tissues. Somatostatin binds to the five distinct receptor (SSTR) subtypes with relatively high and equal affinity for each subtype. Binding to the different types of somatostatin subtypes have been associated with the treatment of various conditions and/or diseases. (“sstr2”) (Raynor, et al., Molecular Pharmacol. 43:838 (1993); Lloyd, et al., Am. J. Physiol. 268:G102 (1995)) while the inhibition of insulin has been attributed to the somatostatin type-5 receptor (“sstr5”) (Coy, et al. 197:366-371 (1993)). Activation of types 2 and 5 have been associated with growth hormone suppression and more particularly growth hormone (“GH”) secreting adenomas (Acromegaly) and TSH secreting adenomas. Activation of type 2 but not type 5 has been associated with treating prolactin secreting adenomas. Other indications associated with activation of the somatostatin receptor subtypes are inhibition of insulin and/or glucagon for treating diabetes mellitus, angiopathy, proliferative retinopathy, dawn phenomenon and nephropathy; inhibition of gastric acid secretion and more particularly peptic ulcers, enterocutaneous and pancreaticocutaneous fistula, irritable bowel syndrome, Dumping syndrome, watery diarrhea syndrome, AIDS related diarrhea, chemotherapy-induced diarrhea, acute or chronic pancreatitis and gastrointestinal hormone secreting tumors; treatment of cancer such as hepatoma; inhibition of angiogenesis, treatment of inflammatory disorders such as arthritis; retinopathy; chronic allograft rejection; angioplasty; preventing graft vessel and gastrointestinal bleeding. It is preferred to have an analog which is selective for the specific somatostatin receptor subtype or subtypes responsible for the desired biological response, thus, reducing interaction with other receptor subtypes which could lead to undesirable side effects.
The development of potent, smaller SRIF agonists led to the discovery of differing affinities of the various truncated ligands for the different subtypes. It appears that Trp8-Lys9 sequence often is present in ligands that are recognized by the receptor. The Trp8-Lys9 sequence forms part of a β-bend which is usually stabilized via substitution of D- for L-Trp, cyclization of the backbone, a disulfide bridge, or all constraints. One unintended consequence of such structural simplification, carried out before the discovery of multiple receptor subtypes, was the loss of broad spectrum binding affinity. This is typified by the high type 2 but low type 1, 3, 4, and 5 affinities of peptides in the OCTREOTIDE® series. Thus, the many basic biological studies with this type of analog failed to detect effects mediated by all but one of the somatostatin receptor types. Since then, much work has gone into the re-introduction of broader spectrum binding into small, biologically stable peptides on the one hand and the development of peptides and peptidomimetics with discrete specificity for a particular receptor.
We have discovered that peptide backbone constraint can be introduced by N-alkylation of individual amino acids. This modification largely restricts the affected residue and the amino acid preceding it to an extended conformation. Thus, additionally blocks potential intramolecular hydrogen bonding sites and also proteolytic enzyme cleavage sites thus potentially enhancing the pharmacokinetic properties of a peptide. Only a few N-methyl amino acids are commercially available and their synthesis is tedious. However, in another aspect of the present invention, we have discovered a procedure to N-methylate truncated somatostatin analogs at every amino acid residue using the solid-phase procedure, adopted from the recent publication reported by Miller and Scanlan.